1. Field of the Invention
The present invention relates to an ethylene polymer, and a process for preparing the same. More particularly, the invention relates to an ethylene polymer having a high melt tension, a high swell ratio, a narrow molecular weight distribution and a small proportion of long-chain branches, and to a process for preparing the same.
2. Discussion of the Related Art
Ethylene polymers have heretofore been widely used as materials of hollow molded articles, extrusion molded articles, films and sheets. The properties required for such ethylene polymers vary depending on the use of the polymers and the molding method thereof. For example, in order to prevent vibration or burst of bubbles at the time of molding inflation films at high speed, or to prevent drawdown at the time of hollow molding or sheet forming, an ethylene polymer having a high melt tension must be used. Further, in order to produce bottles having a good shape of pinch-off scar or hollow molded articles having a narrow thickness distribution by hollow molding, an ethylene polymer having a high swell ratio must be used.
Ethylene polymers prepared by the use of Ziegler-Natta catalysts such as MgCl.sub.2 -carrier support type Ti catalysts are excellent in rigidity and impact resistance because almost no long-chain branch is present therein, but they are inferior in moldability to ethylene polymers prepared by the use of Cr type Phillips catalyst. On the other hand, the ethylene polymers prepared by a high-pressure method or prepared by the use of the Cr type Phillips catalysts have a higher melt tension, higher swell ratio and higher moldability than the ethylene polymers prepared by the use of the Ziegler-Natta catalysts, but they are deteriorated in the rigidity and impact resistance due to the presence of the long-chain branches.
Under these circumstances, there have been made various studies on the ethylene polymers which are prepared by the use of Ziegler-Natta catalysts and are excellent in moldability and the like.
For example, Japanese Patent Laid-Open Publication No. 12735/1980 describes an ethylene polymer which is obtained by blending an ethylene polymer prepared by the use of Ziegler-Natta catalyst with an ethylene polymer prepared by means of a high-pressure method. Further, Japanese Patent Laid-Open Publication No. 36546/1985 describes an ethylene polymer which is obtained by blending an ethylene polymer prepared by the use of Ziegler-Natta catalyst with an ethylene polymer prepared by the use of Cr type Phillips catalyst. However, although thsese ethylene polymers are improved in the moldability, the rigidity and impact strength inherent in the ethylene polymer prepared by the use of Ziegler-Natta catalyst are deteriorated because the proportion of the long-chain branches is increased.
Further, Japanese Patent Laid-Open Publication No. 89341/1984 describes an ethylene polymer which is obtained by modifying an ethylene polymer prepared by the use of Ziegler-Natta catalyst in the presence of a radical initiator; and Japanese Patent Laid-Open Publication No. 164347/1984 describes an ethylene polymer which is obtained by modifying an ethylene polymer prepared by the use of Ziegler-Natta catalyst with maleic acid. Also in these cases, however, although these ethylene polymers improved in the moldability, the rigidity and impact strength thereof are deteriorated because the proportion of the long-chain branches is increased.
Furthermore, Japanese Patent Laid-Open Publications No. 158204/1982 and No. 106806/1985 describe ethylene polymers which are obtained by the use of a specific catalyst, have a small proportion of the long-chain branches and are excellent in the moldability. However, these ethylene polymers have large values of Mw/Mn and Mz/Mw and include a polymer having a molecular weight extremely larger than the average molecular weight. Such a polymer causes poor appearance of its molded article such as fish eye in many cases.
Still further, Japanese Patent Laid-Open Publication No. 130314/1986 describes an ethylene polymer excellent in moldability which is prepared by a multi-stage polymerization process. This ethylene polymer has a broad molecular weight distribution thereby to include an ethylene polymer having a molecular weight extremely larger than the average molecular weight, resulting in poor appearance of the molded article such as fish eye.
Still further, Japanese Patent Laid-Open Publication No. 218507/1992 describes an olefin polymerization catalyst comprising a solid titanium catalyst component I and an organometallic compound catalyst component II as described hereinafter. However, this olefin polymerization catalyst is mainly used for preparing homopolymers or copolymers comprising .alpha.-olefins having 3 or more caron atoms as a main ingredient. Therefore, it has not yet become known that in the case where such an olefin polymerization catalyst is used for polymerizing ethylene, the resulting ethylene polymer is excellent in moldability, and a molded article therefrom is excellent in rigidity and impact resistance without causing poor appearance.
As described above, there have been made earnest researches on the ethylene polymers which are prepared by the use of Ziegler-Natta catalyst, are excellent in moldability and can be molded into articles excellent in mechanical strength and almost free from poor appearance, but any ethylene polymer having such properties was not found.
The present inventors have studied on the ethylene polymers which are prepared by the use of Ziegler-Natta catalyst, excellent in moldability and can be molded into articles excellent in mechanical strength and almost free from poor appearance. As a result, they have found that an ethylene polymer having a density and a melt flow rate in the specific ranges, having a specific relationship between the melt tension (MT) and the melt flow rate (MFR) and having a molecular weight distribution Mw/Mn, a molecular weight distribution Mz/Mw, a g* value indicating a proportion of long-chain branches and a swell ratio all in the specific ranges is excellent in moldability and can be molded into articles excellent in mechanical strength and almost free from poor appearance. The present inventors have also found that such an ethylene polymer can be prepared by the use of a specific catalyst. Based on these findings, the present invention has been accomplished.